An overlay network is known wherein a plurality of logical networks are constructed on a physical network by tunneling. In such an overlay network as just described, a client packet is encapsulated, and an identifier of a logical network is stored into a header of the encapsulated packet. Then, the encapsulated packet is transferred in a logical network corresponding to the identifier included in the header. Each logical network is terminated at a terminating device such as a virtual extensible local area network (VXLAN) tunnel end point (VTEP).
A packet transmission method is known wherein a transmission port from which a packet is to be transmitted is selected based on a hash value calculated from header information of the packet and the packet is transmitted from the selected transmission port. For the calculation of a hash value, for example, the 5-tuple in the header information is used. The 5-tuple includes a transmission source internet protocol (IP) address, a destination IP address, a transmission source port number, a destination port number and a protocol number. The 5-tuple makes it possible to suppress the deviation of transmission paths of flows including a series of packets in the network.
However, in an overlay network in a data center or the like, a packet is transmitted between terminating devices of tunneling. Therefore, pieces of header information of packets transmitted between the same pair of terminating devices are same. Therefore, also hash values calculated from the header information are same as each other, and the transmission paths of packets sometimes deviate. A technology is known wherein, in order to reduce the deviation, the transmission source port number in the header of an encapsulated packet is replaced by a hash value calculated from a client packet. As related art, for example, Japanese Laid-open Patent Publication No. 2004-254132 is disclosed.
Incidentally, when a port from which a packet is to be transmitted is selected based on a hash value calculated from header information of the packet, in the long run, ports are logically selected uniformly, and the deviation of transmission paths of flows is suppressed. However, in the short run, the ports are not necessarily selected uniformly. Therefore, in the short run, selection is concentrated on some of the ports and flows are concentrated on some of the transmission paths in the network in some cases.
In an overlay network, when a hash value calculated from a client packet is stored into the header of an encapsulated packet, the calculation algorithm for a hash value relies upon a hypervisor incorporated in a device that carries out encapsulation. Therefore, a user of the device that performs encapsulation does not know what hash value is calculated from a client packet. It is not known from which port a packet including a client packet is transmitted. Therefore, it is not known what flow distribution is exhibited for each path. From the foregoing, it is desirable to make it possible to achieve precise management of transmission paths of flows such as suppression of deviation of flows.